1. Field of the Invention
The present invention relates to a mobile communication system supplying communication services from a base station to a mobile station such as a cellular phone through a radio channel, and more particularly relates to a mobile communication system in which a mobile station and a plurality of base stations exchange information with each other.
2. Description of the Related Art
A conventional mobile communication system is shown in FIG. 1. A mobile communication system 100 shown in FIG. 1 includes a mobile station 101, base stations 102 through 108, radio network controllers 109 and 110, and a switch station 111. The switch station 111, the radio network controllers 109 and 110, and the base stations 102 through 108 are connected to each other hierarchically. The base stations 102 through 108 form radio zones indicated by broken lines in FIG. 1. The mobile station 101 capable of moving freely between the radio zones is connected to one of the base stations 102 through 108 in the radio zones, for example, the base station 105, by a radio channel. The mobile station 101 and the base station 105 connected to each other by the radio channel execute bi-directional communication through the radio channel. Each base station shown in FIG. 1 communicates with one or more mobile stations through radio channels. Each of the base stations 102, 104 and 107 is connected to the radio network controller 109, and communicates with the radio network controller 109. Similarly, each of the base stations 103, 105, 106 and 108 is connected to the radio network controller 110, and communicates with the radio network controller 110.
Each of the radio network controllers 109 and 110 is connected to a plurality of base stations, and controls switching a base station communicating with the mobile station 101 when the mobile station 101 moves between the radio zones. The mobile station 101 switches the radio channel when one of the radio network controllers 109 and 110 switches the base station with another base station to be connected with the mobile station 101, in other words, when one of the radio network controllers 109 and 110 hands over a communication with the mobile station 101 from the base station to another base station. The switch station 111 is connected to the radio network controllers 109 and 110, and to a plurality of other switch stations or network systems not shown in the figures. The switch station 111 controls a connection between mobile stations located under the switch station 111, and a connection between a mobile station located under the switch station 111 and a terminal station located under another switch station or another network system.
In a conventional mobile communication system, a radio-channel connection between a base station and a mobile station is executed by following a multiple-accessing method such as an FDMA (Frequency Division Multiple Access) method, a TDMA (Time Division Multiple Access) method, and a CDMA (Code Division Multiple Access) method. Accordingly, the conventional mobile communication system can supply both circuit-switched and packet-switched communication services to mobile stations by having the above-described system structure.
In the above-described mobile communication system 100, the mobile station 101 selects a base station by which a received-signal strength of the radio channel between the mobile station 101 and the base station becomes the highest among the base stations, for instance, the base station 105 shown in FIG. 1. The base station 105 selected by the mobile station 101 is referred to as a selected base station. Since directivity of an antenna of the mobile station 101 is low, a signal transmitted from the mobile station 101 reaches a base station, for example, the base station 103 located near the selected base station 105, while the mobile station 101 is communicating with the selected base station 105. The signal is also received by the other base stations in addition to the selected base station 105 especially when the mobile station 101 is positioned at a border of the radio zones.
In a mobile communication system using the CDMA method as a transmission method of a radio channel, a plurality of base stations can receive a signal transmitted from a single mobile station simultaneously by using a plurality of radio channels or spreading codes. For instance, as shown in FIG. 2, a mobile station 113 is communicating simultaneously with the base stations 106 and 108 by using two spreading codes. Each of the base stations 106 and 108 receives a signal transmitted from the mobile station 101, and transmits the signal to the radio network controller 110. The radio network controller 110 combines signals received from the base stations 106 and 108, and transmits a combined signal to the switch station 111. The above-described communication method of receiving signals from a plurality of base stations at a radio network controller is generally referred to as soft handover, or site diversity. Two typical methods among various methods of combining signals from the plurality of base stations will be described below.
In the first method, each base station transmits demodulated bit information to a radio network controller. Subsequently, the radio network controller regenerates a signal starting from a block that does not contain any errors such by an error correction word and an error correction word unit. In the second method, each base station transmits a received soft decision signal with received-signal strength Intensity (RSSI) or a xe2x80x9csignal to noise and interferencexe2x80x9d ratio S/(N+I) to the radio network controller. The radio network controller regenerates a signal by combining the above-mentioned information received from each bases station with a maximum ratio. By following the above-described methods, an error rate of the signal transmitted from a mobile station is reduced by combining signals received from a plurality of base stations at the radio network controller.
Since the radio network controller combines signals received from the plurality of base stations in the conventional mobile communication system, the radio network controllers 109 and 110 cannot communicate simultaneously with a mobile station 112 respectively through the base stations 102 and 105 by the multiple base station concurrent reception method, as shown in FIG. 2. Thus, the conventional mobile communication system cannot continuously supply communication services based on the multiple base station concurrent reception method to a moving mobile station.
Additionally, in the conventional mobile communication system, a processing load at each radio network controller increases with increase in the number of mobile stations by using the multiple base station concurrent reception method in communication except handover. Furthermore, when transmitting signals from a plurality of base stations to a mobile station, in other words, when executing a multiple base station concurrent transmission or a multiple base station in-order transmission, the conventional mobile communication system contains a problem that the system cannot supply constant communication services to the mobile station depending on a position of the mobile station and a problem that a processing load at a communication node uniting the plurality of base stations increases, since the system must control the plurality of base stations all together.
Accordingly, it is a general object of the present invention to provide a mobile communication system in which a mobile station and a plurality of base stations communicate with each other. A more specific object of the present invention is to provide a mobile communication system that enables stable communication between communication nodes such as a mobile station and a plurality of base stations, and minimizes a processing load at each communication node.
The above-described object of the present invention is achieved by a mobile communication system including a plurality of base-stations placed in a communication service area and connected to a network, wherein a mobile station communicates with a second station through the plurality of base stations and the network, the mobile communication system including a first means for determining a virtual base station group, that is, an aggregation of the plurality of base stations communicating with the mobile station, based on a condition of a radio transmission path between the mobile station and each base station, and a second means for combining information received by the plurality of base stations that belong to the virtual base station group at a base station included in the virtual base station group, wherein the network connects the plurality of base stations non-hierarchically, wherein the base station transmits combined information whose destination is the second station to the network.
In the mobile communication system, the plurality of base stations placed in the communication service area are connected non-hierarchically to the network. Additionally, the base stations belonging to the virtual base station group are not limited to specific base stations when the mobile station moves in the communication service area. Accordingly, communication between a moving mobile station and the plurality of base stations is stabilized. Additionally, a single base station belonging to the virtual base station group executes processes such as combination of the information and transmission of the combined information that are related to communication between the mobile station and the second station. Accordingly, execution of the processes related to the communication between the mobile station and the second station is not concentrated in a single communication node, even if the number of base stations placed in the communication service area increases.